1. Field of the Invention The invention is directed to a method for recognizing an off-hook condition of a terminal device at a two-wire subscriber line in a switching center or the like, in which the off-hook condition of a first terminal device working in a different frequency band at the same two-wire subscriber line is recognized by acquiring a loop d.c. current and the comparison thereof to a threshold (event threshold).
2. Description of the Related Art
It is necessary to reliably and unequivocally acquire the off-hook condition of subscriber-side terminal devices in a switching center at which the subscriber lines converge. For two-wire subscriber lines, a subscriber circuit is respectively allocated in the switching center that has an indicator allocated to it with which the off-hook condition of the subscriber-side terminal device is recognized.
This takes place by acquiring the loop current for analog terminal devices such as traditional telephone sets and fax devices. In the on-hook condition, i.e., when the handset is placed on the cradle switch, the terminating impedance of the terminal device is infinitely high (open loop), and thus no current flows via the two-wire line to the exchange circuit at which a voltage is applied.
As soon as the handset is taken from the cradle switch, the terminating impedance is comparatively low (approximately 300 Ω, which causes a loop d.c. current to flow via the two-wire line. This is recognized by an exchange circuit on the basis of an indicator. When this current exceeds a threshold, an off-hook condition is presumed, and all following circuits are activated, particularly the circuit arrangement that triggers the dial event and the connection event, etc.
The same procedure is also fundamentally possible when the subscriber-side terminal device is a digital terminal device that is connected to the two-wire subscriber line via a modem. It has become increasingly desirable to connect not only a single terminal device to a two-wire subscriber line but at least one further terminal device, the pair usually being an analog telephone and a digital terminal device. When the previously standard system of a two-party line is used, it is not possible to simultaneously operate of both terminal devices. However, it is desirable to also be able to transmit data in addition to a traditional telephone call, which is referred to as a “data-over-voice” application. Such applications are especially desirable in ADSL networks.
To achieve this, it was previously necessary to couple the digital terminal device via a POTS splitter and, further, to allocate a filter circuit capable of recognizing and coupling out the high-frequency part to the subscriber circuit at the exchange side. This made use of the fact that analog voice connections occur in a frequency range up to approximately 16 KHz, whereas digital data connections occur in a frequency range above 30 KHz.
Since the recognition of the off-hook condition must be dependable, it is necessary in this application to allocate not only an indicator to each subscriber circuit in the switching center but also a filter or, respectively, some other recognition circuit with respect to the transmission of digital data.
These two recognition circuits, indicator and filter, must be constantly in operation and therefore require the constant supply of electrical power. Furthermore, the resulting heat produced requires additional measures such as aerators or similar mechanisms in order to eliminate this heat. The power consumption, which is referred to as consumed power, lies in the range of less than 1W per subscriber, but must be expended for all subscribers. Since only some of the subscribers connected to the switching center are activated, i.e., are operated in the off-hook condition, a power consumption of some number of watts arises only for such off-hook subscribers.
The power required for a plurality of subscribers connected in the switching center is therefore not negligible. This is reflected in the capital and operating costs and, thus, in the connection charges that every subscriber must pay.